The invention relates to a printing mechanism comprising a printing member which is displaceable along the printing platen, and means for the guiding and feeding of a record carrier which is guided around the printing platen.
Underneath the printing platen, which in office machine printing mechanisms is usually a circular cylindrical drum, there are usually arranged transport rollers which are pressed against the printing platen. For introduction of a record carrier, for example a single form or sets of single forms, the transport rollers are lifted off the printing platen against the force of a spring, so that the forms can be inserted between the printing platen and the rollers. Subsequently, the rollers are lowered again and the record carrier is transported by rotation of the rollers and/or the printing platen. The record carrier is transported to the printing zone through a trough whose shape is adapted to the shape of the printing platen. During further transport the record carrier tends to move tangentially away from the printing platen. In order to ensure that the printing member, moving along the printing platen, does not disturb the introduction of the record carrier, the printing member is usually situated laterally outside the actual printing zone in which the record carrier is to be introduced.
In order to minimize the lifting off of the record carrier from the printing platen, it is known from German Auslegeschrift No. 25 03 642 to extend the already present guide trough as far as over the printing zone; however, it is then necessary to cut out the actual printing zone in the form of a wide window. The further guiding of the record carrier, therefore, is limited to the edge zones of the record carrier. Depending on the thickness of the record carrier, its center also tends to disengage from the printing platen. Moreover, such a construction of the guide trough has the disadvantage that it allows a record carrier to be fed only from behind and not from underneath the printing platen.